As outlined in the article -
"The specs to look for would be near rail-to-rail input and output, along with reduced supply voltage operation and high slew rate. I tried replacing the LM324 with a TL074, but the rectifier formed by two of the op-amp sections stopped working, so I put the original op-amp back (I installed a 14 pin socket, and so was able to check more than one TL074, but none worked in-circuit). The design biases the rectification section asymetrically and rather low (+1.5V from the '-' terminal) and the TL074 probably just won't work this close to the supply rail."

I dabbled around in the Nat Semi website and best I could figure out was either LMV324 or LMV824, but I am not well-versed with chips so I'd like to get it straight from the experts!

Are there any online cross references or selection guides that I could access ?
Thanks.

I think the fellow who wrote it is somewhat confused about the meter. The "c - weighting" is intended to measure the spl within a specific band of frequencies. The meter is intended for general purpose use where one is interested in the noise level produced by machinery, for example, and the weighting defines the bandwidth over which it operates and reads correctly. The modifications he makes to extend the bandwidth mean that it is no longer a "c-weighted" measurement, so the calibration is no longer correct (assuming it was in the first place). He has converted the meter from a c-weighted spl meter to a sort of general sound level meter with no calibration.

I think the intent is to use it for speaker measurement duty - either as a mic-preamp or direct readout of absolute spl. (and not ambient sound/noise level measurement duty)
I imagine you would desire flat response in that case ?

Originally posted by percy
"The specs to look for would be near rail-to-rail input and output, along with reduced supply voltage operation and high slew rate. I tried replacing the LM324 with a TL074, but the rectifier formed by two of the op-amp sections stopped working, so I put the original op-amp back (I installed a 14 pin socket, and so was able to check more than one TL074, but none worked in-circuit). The design biases the rectification section asymetrically and rather low (+1.5V from the '-' terminal) and the TL074 probably just won't work this close to the supply rail."

I dabbled around in the Nat Semi website and best I could figure out was either LMV324 or LMV824, but I am not well-versed with chips so I'd like to get it straight from the experts!

Are there any online cross references or selection guides that I could access ?
Thanks.

LM324 works with lower voltages, than most general Op-amps.
Single supply 3V or +-1.5V.
In fact it works downto a supply of minimum 2.3V.

Input voltage is -0.3 to (+V -1.5) Volt.
So, when using a single +3 volt supply, input can be -0.3 volt to +1.5 Volt.

Hi,
The (formerly Motorola, now On-Semi) MC33204P is a possible candidate.
Its maximum voltage is only 12 volt (or 2x6V) but the rest is better than LM324. Fully rail to rail input and output (RRIO) and about twice the GBW and slewrate.
Might work.
Rick

The LM324 has plenty of gain past 20KHz. If you are getting rolloff below 20K something else is doing it. I see no real reason to replace the LM324 in this application. For work where the signal is NOT going to be heard, the LM324 is my default device because it is very low cost and there is never an issue ib getting supply. Of course, for audio work it sucks big time.

As it is, the frequency response has been tailored to compensate for what is a cheap microphone and changing any of this is more likely to throw off the calibration of the unit. My unit checked out very nicely against a General Radio unit costing 100 times as much.

If you do have an audio circuit using the LM324, the sound quality can be improved a lot by conencting a 6.8K resistor between the output and the negative supply. This biases the class B (no, not class AB) output stage into class A and the open loop distortion drops by a huge amount.

If you do have an audio circuit using the LM324, the sound quality can be improved a lot by conencting a 6.8K resistor between the output and the negative supply. This biases the class B (no, not class AB) output stage into class A and the open loop distortion drops by a huge amount.

Dan

Thanks for that valuable info.

I was trying to figure out why my Preamp I made out of a LM324 was distorting at all volumes, high or low. Class B go figure!

I will connect a resistor to the negative rail and output to add some bias to the output stage. Should I continue to use this chip, or would I be better off building a descrete preamp? Or should I just plug in a better opamp chip in place of the 324?

There are reasons why a discrete pre-amp will be better than an op-amp but your discrete circuit has to be really, really good. If you are talking LM324 level of performance, any decent op-amp will do. Reserve the LM324 for non-audio work, where it really does well.

Just plug in an LM837, MC33078 or even a TL074 or LF347 in place of it. All these have the same pin out. And make sure that in all cases the load impedance is over 1000 ohms and that thre are no capacitors to ground across the output, without a resistor between the output and the capacitors.

although if you want rail-rail i/o then LM6134 (and 6144) seem to be the only choices.

Let us know what happens!

Dan,
just want to make sure whether by non-audio you mean anything that you arn't going to "listen" to (music), or anything related to sine waves in the audible frequency range ? - like this would-be mic preamp for "measurement" duty(not recording/listening duty).
Thanks!